private static void Lift(GotoStatement stmt)
{
AstBlock block = ParentBlock(stmt.Goto);
AstBlock[] path = BackwardsPath(block, ParentBlock(stmt.Label));
AstBlock loopFirstStmt = path[path.Length - 1];
if (loopFirstStmt.Type == AstBlockType.Else)
{
loopFirstStmt = Previous(loopFirstStmt) as AstBlock;
if (loopFirstStmt == null || loopFirstStmt.Type != AstBlockType.If)
{
throw new InvalidOperationException("Found an else without a matching if.");
}
}
AstBlock newBlock = EncloseDoWhile(stmt, block, loopFirstStmt);
block.Remove(stmt.Goto);
newBlock.AddFirst(stmt.Goto);
stmt.IsLoop = false;
}
private static void MoveInward(GotoStatement stmt)
{
AstBlock block = ParentBlock(stmt.Goto);
AstBlock[] path = BackwardsPath(block, ParentBlock(stmt.Label));
for (int index = path.Length - 1; index >= 0; index--)
{
AstBlock child = path[index];
AstBlock last = child;
if (child.Type == AstBlockType.If)
{
// Modify the if condition to allow it to be entered by the goto.
if (!ContainsCondComb(child.Condition, Instruction.LogicalOr, stmt.Condition))
{
child.OrCondition(stmt.Condition);
}
}
else if (child.Type == AstBlockType.Else)
{
// Modify the matching if condition to force the else to be entered by the goto.
if (!(Previous(child) is AstBlock ifBlock) || ifBlock.Type != AstBlockType.If)
{
throw new InvalidOperationException("Found an else without a matching if.");
}
IAstNode cond = InverseCond(stmt.Condition);
if (!ContainsCondComb(ifBlock.Condition, Instruction.LogicalAnd, cond))
{
ifBlock.AndCondition(cond);
}
last = ifBlock;
}
Enclose(block, AstBlockType.If, stmt.Condition, Next(stmt.Goto), last);
block.Remove(stmt.Goto);
child.AddFirst(stmt.Goto);
block = child;
}
}
private static AstBlock Enclose(
AstBlock block,
AstBlockType type,
IAstNode cond,
IAstNode first,
IAstNode last = null)
{
if (first == last)
{
return(null);
}
if (type == AstBlockType.If)
{
cond = InverseCond(cond);
}
// Do a quick check, if we are enclosing a single block,
// and the block type/condition matches the one we're going
// to create, then we don't need a new block, we can just
// return the old one.
bool hasSingleNode = Next(first) == last;
if (hasSingleNode && BlockMatches(first, type, cond))
{
return(first as AstBlock);
}
AstBlock newBlock = new AstBlock(type, cond);
block.AddBefore(first, newBlock);
while (first != last)
{
IAstNode next = Next(first);
block.Remove(first);
newBlock.Add(first);
first = next;
}
return(newBlock);
}
private static void RemoveEmptyBlocks(AstBlock mainBlock)
{
Queue <AstBlock> pending = new Queue <AstBlock>();
pending.Enqueue(mainBlock);
while (pending.TryDequeue(out AstBlock block))
{
foreach (IAstNode node in block)
{
if (node is AstBlock childBlock)
{
pending.Enqueue(childBlock);
}
}
AstBlock parent = block.Parent;
if (parent == null)
{
continue;
}
AstBlock nextBlock = Next(block) as AstBlock;
bool hasElse = nextBlock != null && nextBlock.Type == AstBlockType.Else;
bool isIf = block.Type == AstBlockType.If;
if (block.Count == 0)
{
if (isIf)
{
if (hasElse)
{
nextBlock.TurnIntoIf(InverseCond(block.Condition));
}
parent.Remove(block);
}
else if (block.Type == AstBlockType.Else)
{
parent.Remove(block);
}
}
else if (isIf && parent.Type == AstBlockType.Else && parent.Count == (hasElse ? 2 : 1))
{
AstBlock parentOfParent = parent.Parent;
parent.Remove(block);
parentOfParent.AddAfter(parent, block);
if (hasElse)
{
parent.Remove(nextBlock);
parentOfParent.AddAfter(block, nextBlock);
}
parentOfParent.Remove(parent);
block.TurnIntoElseIf();
}
}
}
private static void MoveOutward(GotoStatement stmt, int gLevel, int lLevel)
{
AstBlock origin = ParentBlock(stmt.Goto);
AstBlock block = origin;
// Check if a loop is enclosing the goto, and the block that is
// directly related to the label is above the loop block.
// In that case, we need to introduce a break to get out of the loop.
AstBlock loopBlock = origin;
int loopLevel = gLevel;
while (loopLevel > lLevel)
{
AstBlock child = loopBlock;
loopBlock = loopBlock.Parent;
loopLevel--;
if (child.Type == AstBlockType.DoWhile)
{
EncloseSingleInst(stmt, Instruction.LoopBreak);
block.Remove(stmt.Goto);
loopBlock.AddAfter(child, stmt.Goto);
block = loopBlock;
gLevel = loopLevel;
}
}
// Insert ifs to skip the parts that shouldn't be executed due to the goto.
bool tryInsertElse = stmt.IsUnconditional && origin.Type == AstBlockType.If;
while (gLevel > lLevel)
{
Enclose(block, AstBlockType.If, stmt.Condition, Next(stmt.Goto));
block.Remove(stmt.Goto);
AstBlock child = block;
// We can't move the goto in the middle of a if and a else block, in
// this case we need to move it after the else.
// IsLoop may need to be updated if the label is inside the else, as
// introducing a loop is the only way to ensure the else will be executed.
if (Next(child) is AstBlock elseBlock && elseBlock.Type == AstBlockType.Else)
{
child = elseBlock;
}
block = block.Parent;
block.AddAfter(child, stmt.Goto);
gLevel--;
if (tryInsertElse && child == origin)
{
AstBlock lBlock = ParentBlock(stmt.Label);
IAstNode last = block == lBlock && !stmt.IsLoop ? stmt.Label : null;
AstBlock newBlock = Enclose(block, AstBlockType.Else, null, Next(stmt.Goto), last);
if (newBlock != null)
{
block.Remove(stmt.Goto);
block.AddAfter(newBlock, stmt.Goto);
}
}
}
}
// This is a modified version of the algorithm presented on the paper
// "Taming Control Flow: A Structured Approach to Eliminating Goto Statements".
public static void Eliminate(GotoStatement[] gotos)
{
for (int index = gotos.Length - 1; index >= 0; index--)
{
GotoStatement stmt = gotos[index];
AstBlock gBlock = ParentBlock(stmt.Goto);
AstBlock lBlock = ParentBlock(stmt.Label);
int gLevel = Level(gBlock);
int lLevel = Level(lBlock);
if (IndirectlyRelated(gBlock, lBlock, gLevel, lLevel))
{
AstBlock drBlock = gBlock;
int drLevel = gLevel;
do
{
drBlock = drBlock.Parent;
drLevel--;
}while (!DirectlyRelated(drBlock, lBlock, drLevel, lLevel));
MoveOutward(stmt, gLevel, drLevel);
gBlock = drBlock;
gLevel = drLevel;
if (Previous(stmt.Goto) is AstBlock elseBlock && elseBlock.Type == AstBlockType.Else)
{
// It's possible that the label was enclosed inside an else block,
// in this case we need to update the block and level.
// We also need to set the IsLoop for the case when the label is
// now before the goto, due to the newly introduced else block.
lBlock = ParentBlock(stmt.Label);
lLevel = Level(lBlock);
if (!IndirectlyRelated(elseBlock, lBlock, gLevel + 1, lLevel))
{
stmt.IsLoop = true;
}
}
}
if (DirectlyRelated(gBlock, lBlock, gLevel, lLevel))
{
if (gLevel > lLevel)
{
MoveOutward(stmt, gLevel, lLevel);
}
else
{
if (stmt.IsLoop)
{
Lift(stmt);
}
MoveInward(stmt);
}
}
gBlock = ParentBlock(stmt.Goto);
if (stmt.IsLoop)
{
EncloseDoWhile(stmt, gBlock, stmt.Label);
}
else
{
Enclose(gBlock, AstBlockType.If, stmt.Condition, Next(stmt.Goto), stmt.Label);
}
gBlock.Remove(stmt.Goto);
}
}